AS/NZS 4234 outlines the modelling methodology for annual performance assessment of solar water heaters (SWHs) and heat pump water heaters (HPWHs).
The results of AS/NZS 4234 modelling are the annual electricity consumption of the water heater, and the percentage energy savings of this water heater compared to a conventional electric storage water heater.
These metrics are used to calculate the number of certificates (STC, VEEC, ESC) that the product would be eligible for under the different Australian federal & state incentive schemes (SRES, VEU, ESS, REPS).
The standard applies to a range of energy efficient water heating systems, applicable to both residential & commercial sites across Australia and New Zealand.
The AS/NZS 4234:2021 version update
The current version of AS/NZS 4234:2021 which superseded the previous AS/NZS 4234:2008. The key updates were:
- Additional technologies added including PV water heating, variable speed heat pumps, commercial systems
- Reference systems energy consumption updated to current MEPS requirements
- Weather files updated
Heat Pump Water Heater Modelling Approach
The standard states how to treat aspects such as:
- Component testing for:
- Solar thermal collectors
- Pumps
- Air-source heat pumps
- Photovoltaic water heaters
- Storage tanks
- Gas heaters
- Performance evaluation including:
- Climate data including ambient air temperature & humidity, solar radiation, wind speed (hourly data is provided)
- Thermal energy loads – daily and seasonal load patterns
- Legionella control
- Minimum delivery temperatures
- Modelling methodologies for water heaters:
- Thermal stratification and mixing
- Piping configurations
- Heat pump flow rate control
- Modelling gas water heating
- Modelling PV water heaters
- Modelling solar-boosted heat pump water heaters
- Modelling commercial systems
- Test report requirements
Modelling heat pump water heaters (HPWHs)
AS/NZS 4234:2021 section 4 specifies how to model HPWHs, including the treatment of water flow rates for water-cycling systems, stand-by power, thermal stratification, and low ambient temperature operation.
The main category separation for water heaters is whether the condenser is integral to the tank (wrap-around coil, microchannel heat exchanger, or immersed coil), or stand-alone to the tank. Whether a system is “integral” or “stand-alone” has a major impact on the AS/NZS 4234 assessment.
The heat pump componentry, including evaporator, compressor, expansion valve may be integrated to the tank as a single unit (“all-in-one”) or be located in a separate unit (“split”). Whether a system is “split” or “all-in-one” does not have any impact on the AS/NZS 4234 assessment.
There are then four main types of HPWHs:
- Split integral: condenser integral to tank with a separate outdoor unit, connected by refrigerant piping
- All-in-one integral: all components together in a single unit, condenser integral to tank
- Split stand-alone: outdoor unit contains the condenser, connected to storage tank with water piping and pump
- All-in-one stand-alone: all components together in a single unit, condenser separate to tank, connected with water piping and pump within the single unit
Component testing is completed according to AS/NZS 5125.1:2014 which provides correlation equations for COP and Power input. These equations are incorporated in the HPWH model to determine thermal capacity at each timestep of the simulation.
For Class A HPWHs, which are capable of operating in low ambient conditions, AS/NZS 5125.1 test condition 5 (low ambient) data is used to determine the low ambient temperature operation penalty, otherwise known as the “frosting penalty”. This penalty de-rates the COP at ambient temperatures below the “initial frosting temperature”, typically 6-9 °C, which is the lowest ambient temperature the HPWH would operate without the evaporator frosting.
The heat pump is configured in the model using physical parameters, such as condenser dimensions, tank size, pump flow rate, control logic.
The heat pump water heater is modelled with small and medium thermal energy loads, with the requirement that the water temperature at the tempering valve outlet remains at least 45 °C for the entire annual simulation. This ensures that the product has been designed appropriately to deliver ample hot water for the consumer.
AS/NZS 4234 reports
AS/NZS 4234 gives requirements on what details are to be included in a modelling report. These include description & schematic of the product modelled, product parameters and test results, control logic description, simulation software used, purchased energy use and calculation of annual energy savings relative to reference water heaters.
All energy efficiency incentive schemes require a copy of this AS/NZS 4234 modelling report to assist auditors with verification of the modelling procedure.
How EnergyAE Can Help
EnergyAE has significant experience with TRNSYS and the AS/NZS 4234 standard. We produce accurate, compliant reports and help you get the best performance out of your system. Our knowledge of how these models work means we can help you to fine-tune designs to improve energy efficiency and thermal performance of your systems.
